This invention relates generally to flowmeters adapted to measure the flow rate of fluids, and more particularly to a flowmeter capable of accurately measuring and transmitting exceptionally low flow rates.
In recent years the need for flowmeters and flow controllers for pilot plants and plants manufacturing such materials as pharmaceuticals and rare chemcials where extremely low flow rates are encountered has aroused considerable interest. Inasmuch as the present invention provides a flowmeter which satisfies this requirement, by way of introduction, we shall briefly review those types of flowmeters which are commonly used for measuring relatively low flow rate and indicate why these meters fail to accurately measure and transmit extremely low rates of flow.
One well-known class of flowmeters is the so-called head meter, which functions by measuring the pressure differential or "head" across a suitable restriction to flow in a pipe conducting the fluid to be metered. This pressure differential may be created by an orifice plate, a Venturi restriction, a capillary tube or other form of primary. Head meters are operable over a wide range of flow rates, from Venturi-type meters handling millions of gallons per hour to meters of the capillary type adapted to measure a few c.c. of liquid per hour.
The pressure differential developed across the primary of a head meter is measured by pressure-responsive secondaries having deflectable metal diaphragms. Such meters suffer from inaccuracies which become particularly troublesome at very low flow rates due to hysteresis in the deflection-versus-differential pressure characteristics inherent in metal diaphragms.
Moreover, response time may extend to minutes or hours when the flow rate is so low that is is only capable of satisfying the volume displacement of the diaphragm in minutes or hours. Capillaries foul easily, or if made so large as to prevent fouling, it may be hours before fluid entering one end emerges at the other.
In the area-type flowmeter, as distinguished from the head meter, one finds a variable orifice and a substantially constant pressure drop, rather than a fixed orifice and a varying pressure drop as a function of flow rate. In the area meter, flow rate is reflected by the changing area of the annular opening through which fluid must pass.
In a standard variable area flowmeter such as that disclosed in the Dettmer U.S. Pat. No. 3,712,134, the vertical tube through which the fluid is conducted in the upward direction is provided with a tapered bore affording a variable cross-sectional area. A weighted float or dragbody disposed in the bore is caused to assume a vertical position representing a condition of equilibrium between the downward gravitational force on the float and the upward force of the fluid flowing past the float through the annular orifice which surrounds it. This position of equilibrium is therefore a function of flow rate--the greater the flow rate, the higher the vertical position of the float.
Variable-area flowmeters cannot, as a practical matter, be used at extremely low flow rates except as a visual indicator without transmission. In order to transmit the position of the light-weight drag body or float, it is usually necessary to attach a long extension rod thereto which is coupled to a magnetic follower or other means to convert the vertical position of the float into a corresponding signal. Because of friction as well as magnetic and other forces which load the float, large errors in drag are experienced that distort the relationship between flow rate and float position and thereby give rise to inaccurate readings. Indeed, at extremely low flow rates, because of friction in the associated position-transmitter, the float is likely to stick so that no reading at all is obtained.
Thus standard flowmeters of the head or variable-area type are incapable of providing accurate measurement with signal transmission at extremely low flow rates. One approach heretofore taken toward accurately measuring extremely low flow rate is that disclosed in the Spencer, U.S. Pat. No. 3,662,598. In Spencer, a ferromagnetic ball positioned within a flow tube is shifted therein in the direction of fluid flow and is returned to its original position by actuating a magnetic return system when the ball intercepts a light beam. This forward and back cycle of ball motion is repeated. The transit time of the ball or its oscillatory frequency is a function of flow rate, thereby serving to indicate flow rate.
In the Spencer instrument, there is no mechanical link between the ball and a secondary, as in the case of a variable-area flowmeter coupled to a transmitter, and the area of the flow tube is uniform throughout its length. Inasmuch as the present invention also makes use of a constant area flow tube in conjunction with a ferromagnetic ball, the Spencer instrument as well as force-type flowmeters in accordance with the present invention will hereafter be referred to as a constant area-ball type flowmeter or by its acronum, CAB.
In a CAB meter of the Spencer type, the flow tube is maintained in a horizontal position; hence there is no gravitational component included in the vectors which determine the ball position, for only magnetic and fluid drag forces act on the ball. Thus when there is an absence of flow or an extremely low flow rate, the ball rests on the lower surface of the glass or plastic flow tube and some degree of friction is encountered which affects the accuracy of the instrument. Any accidental small departure from a truly horizontal position may introduce unwanted gravitational forces of large and random magnitude and direction with respect to the flow direction, causing large zero shifts. Moreover, since the CAB meter of the Spencer type employs a magnet whose force is always horizontal and opposed to the flow direction, this meter is incapable of sensing less than a minimum flow imposed by friction or by small residual magnetism, and cannot detect reversed flow. In other words, it cannot have a live zero.